The present invention relates to machinery with sliding or rotating components such as a screw type compressor or supercharger. It addresses specific combinations of materials, arrangement of components, and methods to compensate for thermal expansion. The invention improves size and weight of the assembly. Components can be assembled at room temperature while a better fit is created at operating temperature.
Using an aluminum housing in, for example, a screw compressor to decrease compressor weight is highly desirable but has proven to be problematic. If, for example, steel rotors and/or rotor shafts are used in a typical aluminum housing, the different coefficient of thermal expansion (CTE) rates between such materials can introduce undesirable rotor-to-housing clearances and/or substantial changes in bearing loads. One object of my invention is to use a combination of alloys to optimize CTE effects.
Another object of my invention is to provide an improved way of transferring torque between components so that these components can be joined or assembled at room temperature with the joint between them improving (i.e., becoming tighter) as the compressor temperature transitions to that of normal operation. I have found that this can be achieved by allowing the joint to float axially during the temperature transition period, thereby eliminating axial component loading from CTE differences from the drive mounting mechanisms.
When the appropriate materials are used in accordance with my invention, assembly of a component positioned by interference fit (such as an inner radial load bearing race on rotor and/or rotor shaft) can be accomplished by submerging the rotor/rotor shaft in liquid nitrogen. This extreme cold condition can quickly shrink the rotor/rotor shaft to create sufficient clearance for installation of the interference fit component over the shaft without significant permanent changes in material properties.
Still another object of my invention is to further reduce compressor size by including the drive joint connection feature in the drive shaft itself to allow precise positioning of rotational components while sharing a common bearing. Concentricity of coupled components is very critical in high rotational speed machines. This method decouples concentricity control and torque transfer to separate features, thus improving manufacturability. In addition to reduced size by eliminating independent support bearing(s) from one end of the drive shaft, my invention has the advantage of reducing complexity in the lubrication system by eliminating said bearing. Alignment details between drive and driven components is simplified by combining the support of two mating shaft ends with a single bearing.